1. Field of the Invention
The present invention relates generally to an exposure apparatus suitable for use in manufacturing semiconductor integrated circuits and, more particularly, to position control provided over a stage which carries a photosensitive substrate such as a semiconductor wafer.
2. Related Background Art
In the manufacture of semiconductor integrated circuits, an exposure apparatus of the reduced projection type utilizing a step-and-repeat system, that is, a stepper, is employed as the main device in a lithography step. The stepper includes an alignment apparatus which serves to position a projected image of a pattern formed on a mask or a reticle (hereinafter referred to simply as "reticle") in alignment with a circuit pattern (hereinafter referred to as "chip") formed on a photosensitive substrate (hereinafter referred to as "wafer"). Two major types of alignment apparatus are available; one type is called the on-axis alignment system and the other type the off-axis alignment system. In the on-axis alignment system, a mark on a reticle and a corresponding mark on a wafer are detected by means of a projection lens and the reticle so as to effect alignment, whereas, in the off-axis alignment system, only a mark on a wafer is detected for the purpose of effecting alignment; hence a mark on a reticle is not detected.
In a stepper of the type which utilizes the off-axis alignment system of these two major alignment systems, the mark-detecting reference position of the alignment system, i.e., an alignment position, differs from the projecting position at which an image of the circuit pattern of a reticle is to be projected, i.e., an exposure position. For this reason, conventional apparatus commonly have a construction such as that shown in FIG. 6. As illustrated, laser interferometers 106 and 109 for detecting the respective X- and Y-direction positions of a wafer stage 103 are disposed so that the optical axes of the laser beams generated by the respective laser interferometers 106 and 109 intersect each other at right angles in a common plane and so that the optical axis AX of a projection lens 100 passes through the intersection. Off-axis alignment systems 101 and 102 for optically detecting alignment marks or the like on a wafer W are disposed on the measurement axes of the respective laser interferometers 106 and 109 s that corresponding Abbe errors can be made approximately zero with respect to the alignment position and the exposure position. The laser interferometer 106 is arranged first to irradiate with a laser beam a movable mirror 104 provided on a wafer stage 103 and a fixed mirror 105 provided on the lens barrel of the projection lens 100 and then to photoelectrically detect interference fringes which are formed on the light receiving surface of a receiver by a reflected light beam. Similarly, the laser interferometer 109 is arranged first to irradiate with a laser beam a movable mirror 107 provided on the wafer stage 103 and a fixed mirror 108 provided on the lens barrel of the projection lens 100 and then to photoelectrically detect interference fringes which are formed on the light receiving surface of a receiver by a reflected light beam. In the stepper having the above-described construction and arrangement, alignment marks formed in association with a particular chip on the wafer W are detected by means of the respective alignment systems 101 and 102, and the X- and Y-direction alignment positions of the chip are read from the respective laser interferometers 106 and 109. Then, the wafer stage 103 is caused to travel by a predetermined amount on the basis of the alignment positions, that is, by an amount corresponding to a so-called base line which represents the relative positional relationship between the alignment position and the exposure position. The chip is moved to a location below the projection lens 100 so that, at this position, the projected image of the circuit pattern of the reticle and the chip are accurately super-imposed on each other for the purpose of exposure.
In an apparatus of the type shown in FIG. 6 in which the alignment systems 101 and 102 are disposed at locations offset from the measurement axes of the respective laser interferometers 106 and 109, these alignment systems 101 and 102 are used to detect alignment marks on the wafer W and the X- and Y-direction alignment positions of a particular chip are read from the respective laser interferometers 106 and 109. However, the measured value thus detected may include a measurement error due to the Abbe error. To cope with this measurement error, such an apparatus utilizes a differential interferometer 110 (FIG. 7) which is arranged to irradiate the movable mirror 104 with two divided laser beams, coaxially combine reflected light beams, receive the coaxially combined light beams by means of a receiver, photoelectrically detect interference fringes formed on the light receiving surface of the receiver, and detect the amount of rotation (the amount of yawing) of the wafer stage 103. On the basis of the obtained amount of rotation, the measured values of the respective laser interferometers 106 and 109 are corrected, and the wafer stage 103 is caused to travel on the basis of the corrected values. Subsequently, when the chip is to be moved to a location below the projection lens, positioning of the chip is carried out with the amount of rotation of the wafer stage 103 taken into account, whereby the projected image of the circuit pattern is accurately superimposed on the chip for exposure purposes.
However, steppers provided with off-axis alignment systems of the type described, such as the stepper shown in FIG. 6, involve the following problem. X- and Y-direction alignment marks must be independently detected by means of the alignment systems 101 and 102, so that it takes a substantial time to detect the positions of these alignment marks and also to measure a corresponding base line. As a result, the rate of throughput decreases. On the other hand, steppers of the type provided with the differential interferometer 110 shown in FIG. 7 involve the problem that, since the speed at which the differential interferometer 110 detects the amount of rotation of the wafer stage 103 is slower than the speed at which the wafer stage 103 fluctuates, it is impossible to accommodate fine variations in the position of the wafer stage 103. Moreover, the interval between the two divided laser beams with which the differential interferometer 110 irradiates the movable mirror 104 is restricted by the relationship between the stroke of the wafer stage 103 and the length of the movable mirror 104, with the result that the accuracy with which the differential interferometer 110 detects the amount of rotation of the wafer stage 103 cannot be improved beyond a certain limit.